Receptor tyrosine kinases are critical components of signal transduction pathways which regulate cell proliferation and differentiation. The broad objective of this proposal is to elucidate the role of Cek5, a receptor tyrosine kinase of the Eph family, in the differentiation of retinal neurons. Cek5, based on its spatiotemporal expression, is likely to be heavily implicated in the development and maintenance of the vertebrate nervous system. Cek5 is highly expressed and catalytically active in the neural retina during differentiation. Its localization to axons and growth cones of retinal neurons and its polarized distribution in the developing retina are distinctive compared to other known receptor tyrosine kinases present in the eye. This suggests an active role for Cek5 in neurite outgrowth and pathfinding and as a positional marker of retinal neurons. As a consequence, mutations of the Cek5 gene are likely to disrupt normal retina development. To address the functions of Cek5 in the retina, its intracellular target molecules will be identified by screening a retinal cDNA expression library with a tyrosine- phosphorylated Cek5 receptor probe. The phosphorylated tyrosine residues in the Cek5 cytoplasmic domain which constitute binding sites for the SH2 domains of specific signaling molecules, will be identified in vitro . Whether specific Cek5-target associations are relevant to neurite outgrowth will be determined in vivo, using a transfected PC12 cell system.